1. Field of the Invention
The present invention relates to a method for detecting an abnormality in a hydraulic system of a molding machine and an apparatus therefor. In particular, the present invention relates to a method which is suitable for detecting an abnormality in a hydraulic system which is used for transferring an injection screw of an injection molding machine or for transferring a movable platen having a movable mold mounted thereon, and relates to an apparatus therefor.
2. Description of Related Art
Conventionally, for example, a hydraulic system having a hydraulic cylinder is used as a driving member for reciprocating a movable member, e.g., an injection screw in an injection molding machine, a movable platen having a movable mold thereon or the like.
A hydraulic system having a structure shown in FIG. 4, which is applied for the injection screw of the injection molding machine, is generally known.
Hereinafter, the injection molding machine 1 and the hydraulic system thereof will be explained as follows. The injection molding machine 1 comprises an injection screw 2, a heating tube 3, a hydraulic cylinder 4 of a double rod type for rotating and reciprocating the injection screw 2, a servo valve 7 for supplying a hydraulic fluid into a pair of pressure chambers 5 and 6 which are formed in the hydraulic cylinder 4, pressure sensors 8 and 9 for detecting the pressures in the pair of pressure chambers 5 and 6, and a servo control member 12 for controlling the operation of the servo valve 7. The injection screw 2 is rotatably provided and reciprocated in the heating tube 3. The hydraulic cylinder 4 is connected with the rear end of the injection screw 2. The pressure sensors 8 and 9 are attached to the hydraulic cylinder. The servo control member 12 previously sets a target value of the transfer velocity of the injection screw 2 and a target value of the injection pressure thereof, to be controlled, and controls the operation of the servo valve 7 on the basis of detecting signals from the pair of pressure sensors 8 and 9, and on the basis of a position detecting signal for the hydraulic cylinder 4 from a position sensor 11 which is provided on a injection ram 10 of the hydraulic cylinder 4. The hydraulic system comprises the hydraulic cylinder 4, the servo valve 7, the pair of pressure sensors 8 and 9, and the servo control member 12.
[0004]
The servo control member 12 comprises a servo sequential control unit 13, a velocity conversion unit 14, a velocity operation unit 15, a comparator 19, a pressure operation unit 20, and a servo amplifier 21. The servo sequential control unit 13 previously sets target values of an injection velocity, an injection pressure and the like which constitute injection conditions. The velocity conversion unit 14 calculates the current transfer velocity of the injection screw 2 on the basis of the detecting signal from the position sensor 11. The velocity operation unit 15 compares a signal from the velocity conversion unit 14 which is a feedback signal of the velocity, with the velocity signal having the preset target value which is outputted from the servo sequential control unit 13, and outputs a velocity correction signal on the basis of the comparison result thereof. The detecting signals from the pair of pressure sensors 8 and 9 are applied to the comparator 19 through differential amplifiers 17 and 18. The pressure operation unit 20 compares a signal outputted from the comparator 19 which is a feedback signal of the injection pressure, with a pressure signal having the preset target value which is outputted from the servo sequential control unit 13, and outputs a pressure correction signal on the basis of the result thereof. The servo amplifier 21 which is selectively connected with the pressure operation unit 20 and the velocity operation unit 15, amplifies each of the correction signals from these units and outputs the amplified signal thereof to the servo valve 7 as a driving signal therefor.
The servo valve 7 is connected with a pressure source 22 for supplying the hydraulic fluid to the servo valve 7 and with a drain tank 23 for receiving hydraulic fluid drained from the hydraulic cylinder 4.
In such a conventional hydraulic system, if an injection were carried out in situations where an abnormality occurs in the hydraulic system, there is a possibility of faulty operation of the injection molding machine 1, so that there is a certain danger of destruction of the injection molding machine 1, a mold or the like. The abnormality occurs in the hydraulic system, for example, when either of the pair of pressure sensors 8 and 9 is damaged, when no pressure can be applied to the pressure sensors 8 and 9, or when trouble occurs in the operation of the servo valve 7, for some reason.
Generally, almost the above-mentioned abnormality has been detected after an injection is commenced. It is necessary to detect an abnormality condition before the starting of an injection.